This invention relates to printing sheets, more particularly, those for use in offset printing.
Recently, there has been increasing demands for printing various thermoplastic resin films represented by polyester films according as diversified development of printing. It has also been desired to print on special sheet-form materials such as metal foils, metal-deposited paper, metal fiber sheets, ceramics sheets and the like. Generally, these sheet-form materials have been printed by offset printing, gravure printing, flexographic printing, screen printing, relief printing, or the like printing methods. Among the conventional printing methods, offset printing has attracted much attention since it is easy to make plates, it can be performed at low costs, it provides more beautiful printings as compared other printing methods, it can be performed economically when only a small number of sheets are to be printed, and for some other reasons. Development of printing sheets suitable for offset printing has been made accordingly.
The properties that are required for the printing sheets for use in offset printing include sheet running property, anti-blocking property, anti-setoff property, ink-drying property, ink adhesion property, anti-static property, etc. These properties are not satisfied by sheets themselves; the printing sheets are usually subjected to surface treatment before they can be useful as printing sheets suitable for offset printing. For example, polyester films are coated with a coating material containing an inorganic pigment in order to improve ink-drying property, they are subjected to corona discharge treatment in order to improve ink-adhesion property, or the films themselves are molded from resins which comprise copolymers of the ester monomer with other polar monomers or those which are blends of the polyester with other resins composed of polar monomers. For preventing static charges, countermeasure is taken, for example, by kneading an anti-static agent in the resin before molding into sheets or coating the anti-static agent on the surface of the resin sheets.
However, notwithstanding the above-described treatments, the requirements for printing sheets for use in offset printing are not always satisfied completely from the practical viewpoint. Furthermore, it has heretofore been performed to bond an undertaper to the rear side of the printing sheet to obtain printing sheets for use in offset printing in order to overcome disadvantages of the conventional printing sheets such as poor sheet running property, poor anti-blocking property, poor anti-offset of ink, and the like. In this case, however, other problems arise from the viewpoints of operability and economics in that additional steps are necessitated such as a step of bonding in which the undertaper is bonded to the printing sheets, and a step of stripping in which the undertaper is stripped off from the printing sheets after the printing is over, and also that additional cost for the undertaper is incurred.
In addition, as the result of recent development of ultraviolet-curing ink (hereafter, sometimes referred to as "UV ink", for brevity) which dries in a very short time and particularly advantageous in the operability, printing methods using UV ink has prevailed widely. However, when the above-described sheet-form materials are printed with UV ink, no satisfactory adhesion is obtained between the UV ink after curing and the surface of the sheet material, resulting in peeling off of the printed layer. Therefore, it has been desired to develop printing sheets having improved adhesion between the surface thereof and the ink to be applied.